Found this online regarding Usain Bolt. "At 40 yards of the actual Olympic 100m race, Bolt was at 4.35 or 4.36. His reaction time was .165. My understanding is that the combine 40 is timed from the runner’s actual start rather than from a gun. So if this were in an NFL combine setting, that reaction time would be gone and he’d be at 4.19."
Therefore, Dexter and Usain Bolt are exactly the same speed!!